starting to exercise while pregnant ?

I never been the person to exercise a lot but I did do stretches and walks and hikes a few times a month . Well since I've became pregnant with twins (23 weeks now) I completely stopped everything but now that I'm gaining weight and looking more "pluffed" I want to start toning out or start doing something . I'm NOT wanting to loose weight , just maybe try to tone my arms and butt and legs. Is it possible ? Or too late in the pregnancy to start .  I have no issues in my pregnancy so far and don't want to cause any . Any info will be helpful .